Polarity Reversing Lead

ABSTRACT

A system, including: an implantable neural stimulator including electrodes, at least one antenna and an electrode interface; a radio-frequency (RF) pulse generator module comprising an antenna module configured to send an input signal to the antenna in the implantable neural stimulator through electrical radiative coupling, the input signal containing electrical energy and polarity assignment information that designates polarity assignments of the electrodes in the implantable neural stimulator; and wherein the implantable neural stimulator is configured to: control the electrode interface such that the electrodes have the polarity assignments designated by the polarity assignment information, create one or more electrical pulses suitable for modulation of neural tissue using the electrical energy contained in the input signal, and supply the electrical pulses to the electrodes through the electrode interface such that the electrodes apply the electrical pulses to the neural tissue with the polarity assignments designated by the polarity assignment information.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/694,330, filed Mar. 8, 2018, which is a continuation of U.S. patentapplication Ser. No. 14/068,828, filed Oct. 31, 2013, now U.S. Pat. No.9,757,571, issued Sep. 12, 2017, which is a divisional application ofU.S. patent application Ser. No. 13/562,221, filed Jul. 30, 2012, nowU.S. Pat. No. 9,199,089, issued Dec. 1, 2015, which claims benefit ofU.S. provisional Patent Application 61/513,397, filed Jul. 29, 2011, andis a continuation-in-part of PCT Application PCT/US2012/023029, filedJan. 27, 2012, which claims benefit of U.S. provisional PatentApplication 61/437,561, filed Jan. 28, 2011, all of which are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

This description is related to implanted neural stimulators.

BACKGROUND

Neural modulation of neural tissue in the body by electrical stimulationhas become an important type of therapy for chronic disablingconditions, such as chronic pain, problems of movement initiation andcontrol, involuntary movements, dystonia, urinary and fecalincontinence, sexual difficulties, vascular insufficiency, heartarrhythmia and more. Electrical stimulation of the spinal column andnerve bundles leaving the spinal cord was the first approved neuralmodulation therapy and been used commercially since the 1970s. Implantedelectrodes are used to pass pulsatile electrical currents ofcontrollable frequency, pulse width and amplitudes. Two or moreelectrodes are in contact with neural elements, chiefly axons, and canselectively activate varying diameters of axons, with positivetherapeutic benefits. A variety of therapeutic intra-body electricalstimulation techniques are utilized to treat neuropathic conditions thatutilize an implanted neural stimulator in the spinal column orsurrounding areas, including the dorsal horn, dorsal root ganglia,dorsal roots, dorsal column fibers and peripheral nerve bundles leavingthe dorsal column or brain, such as vagus-, occipital-, trigeminal,hypoglossal-, sacral-, and coccygeal nerves.

SUMMARY

In one aspect, an implantable neural stimulator includes one or moreelectrodes, at least one antenna, and one or more circuits connected toat least one antenna. The one or more electrodes are configured to applyone or more electrical pulses to excitable tissue. The antenna isconfigured to receive one or more input signals containing polarityassignment information and electrical energy, with the polarityassignment information designating polarities for each of theelectrodes. The one or more circuits are configured to control anelectrode interface such that the electrodes have the polaritiesdesignated by the polarity assignment information; create one or moreelectrical pulses using the electrical energy contained in the inputsignal; and supply the one or more electrical pulses to the one or moreelectrodes through the electrode interface such that the one or moreelectrodes apply the one or more electrical pulses to excitable tissueaccording to the polarities designated by the polarity assignmentinformation.

Implementations of this and other aspects may include the followingfeatures. The polarities designated by the polarity assignmentinformation may include a negative polarity, a positive polarity, or aneutral polarity. The electrical pulses include a cathodic portion andan anodic portion. The electrode interface may include a polarityrouting switch network. The polarity routing switch network may includea first input that receives the cathodic portion of the electricalpulses and a second input that receives the anodic portion of theelectrical pulses. The polarity routing switch network may be configuredto route the cathodic portion to electrodes with a negative polarity,route the anodic portion to electrodes with a positive polarity, anddisconnect electrodes with a neutral polarity from the electricalpulses.

The one or more circuits may include a register with an output coupledto a selection input of the polarity routing switch network. Theregister may be configured to store the polarity assignment informationand send the stored polarity assignment information from the registeroutput to the selection input of the polarity routing switch network tocontrol the polarity routing switch network to route the cathodicportion to electrodes with a negative polarity, route the anodic portionto electrodes with a positive polarity, and disconnect electrodes with aneutral polarity from the electrical pulses.

The one or more circuits include a power-on reset circuit and acapacitor, wherein the capacitor may store a charge using a portion ofthe electrical energy contained in the one or more input signals, andwherein the capacitor may be configured to energize the power-on resetcircuit to reset the register contents when the implanted neuralstimulator loses power.

The at least one antenna may be configured to transmit, to the separateantenna through electrical radiative coupling, one or more stimulusfeedback signals. The one or more circuits may be configured to generatea stimulus feedback signal. The stimulus feedback signal may indicateone or more parameters associated with the one or more electrical pulsesapplied to the excitable tissue by the one or more electrodes. Theparameters may include the power being delivered to the tissue and animpedance at the tissue.

The one or more circuits may include a current sensor configured tosense an amount of current being delivered to the tissue and a voltagesensor configured to sense a voltage being delivered to the tissue. Thecurrent sensor may include a resistor placed in serial connection withan anodic branch of the polarity routing switch network, and the anodicportion of the electrical pulses may be transported over the anodicbranch. The current sensor and the voltage sensor are coupled to ananalog controlled carrier modulator, the modulator being configured tocommunicate the sensed current and voltage to the separate antenna.

The at least one antenna may include a first antenna and a secondantenna. The first antenna may be configured to receive an input signalcontaining the electrical energy. The second antenna may be configuredto transmit the stimulus feedback signal to the separate antenna throughelectrical radiative coupling. The second antenna may be furtherconfigured to receive an input signal containing the polarity assignmentinformation. The transmission frequency of the second antenna may behigher than a resonant frequency of the first antenna. The transmissionfrequency of the second antenna may be a second harmonic of the resonantfrequency of the first antenna. The transmission frequency and theresonant frequency are in a range from about 300 MHz to about 6 GHz. Theat least one antenna may be between about 0.1 mm and about 7 cm inlength and between about 0.1 mm to about 3 mm in width. The at least oneantenna may be a dipole antenna.

The one or more circuits may additionally include a rectifying circuitconfigured to rectify the input signal received by the first antenna togenerate the one or more electrical pulses. The rectifying circuit maybe coupled to a RC-timer to shape the one or more electrical pulses. Therectifying circuit may include at least one full wave bridge rectifier.The full wave bridge rectifier may include several diodes, each of whichmay be less than 100 micrometers in length.

In another aspect, system includes a RF pulse generator module. The RFpulse generator module includes an antenna module and one or morecircuits coupled to the antenna module.

The antenna module is configured to send one or more input signals to atleast one antenna in an implantable neural stimulator through electricalradiative coupling. The one or more input signal contain electricalenergy and polarity assignment information that designates polarityassignments of one or more electrodes in the implantable neuralstimulator. The implantable neural stimulator is configured to controlan electrode interface such that the electrodes have the polaritiesdesignated by the polarity assignment information, create one or moreelectrical pulses suitable for stimulation of neural tissue using theelectrical energy contained in the input signal, and supply the one ormore electrical pulses to the one or more electrodes through theelectrode interface such that the one or more electrodes apply the oneor more electrical pulses to neural tissue with the polaritiesdesignated by the polarity assignment information. The antenna module isfurther configured to receive one or more signals from the at least oneantenna in an implantable neural stimulator through the electricalradiative coupling.

The one or more circuits are configured to generate the one or moreinput signals and send the one or more input signals to the antennamodule; extract a stimulus feedback signal from one or more signalsreceived by the antenna module, the stimulus feedback signal being sentby the implantable neural stimulator and indicating one or moreparameters of the one or more electrical pulses; and adjust parametersof the input signal based on the stimulus feedback signal.

Implementations of this and other aspects may include the followingfeatures. The antenna module may be configured to transmit portions ofthe input signal containing electrical energy using a different carrierfrequency than portions of the input signal containing informationencoding the polarity assignments of one or more electrodes.

The antenna module may include a first antenna configured to operate ata first frequency to transmit an input signal containing the electricalenergy and a second antenna configured to operate at a second frequencyto receive the one or more signals from the at least one antenna of theimplantable neural stimulator. The second frequency may be, for example,a second harmonic frequency of the first frequency.

Various implementations may be inherently low in cost compared toexisting implantable neural modulation systems, and this may lead towider adoption of neural modulation therapy for patients in need as wellas reduction in overall cost to the healthcare system.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 depicts a high-level diagram of an example of a wireless neuralstimulation system.

FIG. 2 depicts a detailed diagram of an example of the wireless neuralstimulation system.

FIG. 3 is a flowchart showing an example of the operation of thewireless neural stimulator system.

FIG. 4 depicts a flow chart showing an example of the operation of thesystem when the current level at the electrodes is above the thresholdlimit.

FIG. 5 is a diagram showing examples of signals that may be used todetect an impedance mismatch.

FIG. 6 is a diagram showing examples of signals that may be employedduring operation of the wireless neural stimulator system.

FIG. 7 is a flow chart showing a process for the user to control theimplantable wireless neural stimulator through an external programmer inan open loop feedback system.

FIG. 8 is another example flow chart of a process for the user tocontrol the wireless stimulator with limitations on the lower and upperlimits of current amplitude.

FIG. 9 is yet another example flow chart of a process for the user tocontrol the wireless neural stimulator through preprogrammed parametersettings.

FIG. 10 is still another example flow chart of a process for a lowbattery state for the RF pulse generator module.

FIG. 11 is yet another example flow chart of a process for aManufacturer's Representative to program the implanted wireless neuralstimulator.

FIG. 12 is a circuit diagram showing an example of a wireless neuralstimulator.

FIG. 13 is a circuit diagram of another example of a wireless neuralstimulator.

FIG. 14 is a block diagram showing an example of control and feedbackfunctions of a wireless implantable neural stimulator.

FIG. 15 is a schematic showing an example of a wireless implantableneural stimulator with components to implement control and feedbackfunctions.

FIG. 16 shows an example of a pulse waveform seen at the powermanagement circuitry of a wireless implantable neural stimulator.

FIG. 17 is a schematic of an example of a polarity routing switchnetwork.

FIGS. 18A and 18B, respectively show an example of a waveform generatedby a rectifying circuit of a wireless neural stimulator and thecorresponding spectrum.

FIG. 19 is a flow chart illustrating an example of operations of controland feedback functions of a wireless implantable neural stimulator.

DETAILED DESCRIPTION

In various implementations, a neural stimulation system may be used tosend electrical stimulation to targeted nerve tissue by using remoteradio frequency (RF) energy with neither cables nor inductive couplingto power the passive implanted stimulator. The targeted nerve tissuesmay be, for example, in the spinal column including the spinothalamictracts, dorsal horn, dorsal root ganglia, dorsal roots, dorsal columnfibers, and peripheral nerves bundles leaving the dorsal column orbrainstem, as well as any cranial nerves, abdominal, thoracic, ortrigeminal ganglia nerves, nerve bundles of the cerebral cortex, deepbrain and any sensory or motor nerves.

For instance, in some implementations, the neural stimulation system mayinclude a controller module, such as an RF pulse generator module, and apassive implanted neural stimulator that contains one or more dipoleantennas, one or more circuits, and one or more electrodes in contactwith or in proximity to targeted neural tissue to facilitatestimulation. The RF pulse generator module may include an antenna andmay be configured to transfer energy from the module antenna to theimplanted antennas. The one or more circuits of the implanted neuralstimulator may be configured to generate electrical pulses suitable forneural stimulation using the transferred energy and to supply theelectrical pulses to the electrodes so that the pulses are applied tothe neural tissue. For instance, the one or more circuits may includewave conditioning circuitry that rectifies the received RF signal (forexample, using a diode rectifier), transforms the RF energy to a lowfrequency signal suitable for the stimulation of neural tissue, andpresents the resulting waveform to an electrode array. The one or morecircuits of the implanted neural stimulator may also include circuitryfor communicating information back to the RF pulse generator module tofacilitate a feedback control mechanism for stimulation parametercontrol. For example, the implanted neural stimulator may send to the RFpulse generator module a stimulus feedback signal that is indicative ofparameters of the electrical pulses, and the RF pulse generator modulemay employ the stimulus feedback signal to adjust parameters of thesignal sent to the neural stimulator.

FIG. 1 depicts a high-level diagram of an example of a neuralstimulation system. The neural stimulation system may include four majorcomponents, namely, a programmer module 102, a RF pulse generator module106, a transmit (TX) antenna 110 (for example, a patch antenna, slotantenna, or a dipole antenna), and an implanted wireless neuralstimulator 114. The programmer module 102 may be a computer device, suchas a smart phone, running a software application that supports awireless connection 114, such as Bluetooth®. The application can enablethe user to view the system status and diagnostics, change variousparameters, increase/decrease the desired stimulus amplitude of theelectrode pulses, and adjust feedback sensitivity of the RF pulsegenerator module 106, among other functions.

The RF pulse generator module 106 may include communication electronicsthat support the wireless connection 104, the stimulation circuitry, andthe battery to power the generator electronics. In some implementations,the RF pulse generator module 106 includes the TX antenna embedded intoits packaging form factor while, in other implementations, the TXantenna is connected to the RF pulse generator module 106 through awired connection 108 or a wireless connection (not shown). The TXantenna 110 may be coupled directly to tissue to create an electricfield that powers the implanted neural stimulator module 114. The TXantenna 110 communicates with the implanted neural stimulator module 114through an RF interface. For instance, the TX antenna 110 radiates an RFtransmission signal that is modulated and encoded by the RF pulsegenerator module 110. The implanted wireless neural stimulator module114 contains one or more antennas, such as dipole antenna(s), to receiveand transmit through RF interface 112. In particular, the couplingmechanism between antenna 110 and the one or more antennas on theimplanted neural stimulation module 114 is electrical radiative couplingand not inductive coupling. In other words, the coupling is through anelectric field rather than a magnetic field.

Through this electrical radiative coupling, the TX antenna 110 canprovide an input signal to the implanted neural stimulation module 114.This input signal contains energy and may contain information encodingstimulus waveforms to be applied at the electrodes of the implantedneural stimulator module 114. In some implementations, the power levelof this input signal directly determines an applied amplitude (forexample, power, current, or voltage) of the one or more electricalpulses created using the electrical energy contained in the inputsignal. Within the implanted wireless neural stimulator 114 arecomponents for demodulating the RF transmission signal, and electrodesto deliver the stimulation to surrounding neuronal tissue.

The RF pulse generator module 106 can be implanted subcutaneously, or itcan be worn external to the body. When external to the body, the RFgenerator module 106 can be incorporated into a belt or harness designto allow for electric radiative coupling through the skin and underlyingtissue to transfer power and/or control parameters to the implantedneural stimulator module 114, which can be a passive stimulator. Ineither event, receiver circuit(s) internal to the neural stimulatormodule 114 can capture the energy radiated by the TX antenna 110 andconvert this energy to an electrical waveform. The receiver circuit(s)may further modify the waveform to create an electrical pulse suitablefor the stimulation of neural tissue, and this pulse may be delivered tothe tissue via electrode pads.

In some implementations, the RF pulse generator module 106 can remotelycontrol the stimulus parameters (that is, the parameters of theelectrical pulses applied to the neural tissue) and monitor feedbackfrom the wireless neural stimulator module 114 based on RF signalsreceived from the implanted wireless neural stimulator module 114. Afeedback detection algorithm implemented by the RF pulse generatormodule 106 can monitor data sent wirelessly from the implanted wirelessneural stimulator module 114, including information about the energythat the implanted wireless neural stimulator module 114 is receivingfrom the RF pulse generator and information about the stimulus waveformbeing delivered to the electrode pads. In order to provide an effectivetherapy for a given medical condition, the system can be tuned toprovide the optimal amount of excitation or inhibition to the nervefibers by electrical stimulation. A closed loop feedback control methodcan be used in which the output signals from the implanted wirelessneural stimulator module 114 are monitored and used to determine theappropriate level of neural stimulation current for maintainingeffective neuronal activation, or, in some cases, the patient canmanually adjust the output signals in an open loop control method.

FIG. 2 depicts a detailed diagram of an example of the neuralstimulation system. As depicted, the programming module 102 may compriseuser input system 202 and communication subsystem 208. The user inputsystem 221 may allow various parameter settings to be adjusted (in somecases, in an open loop fashion) by the user in the form of instructionsets. The communication subsystem 208 may transmit these instructionsets (and other information) via the wireless connection 104, such asBluetooth or Wi-Fi, to the RF pulse generator module 106, as well asreceive data from module 106.

For instance, the programmer module 102, which can be utilized formultiple users, such as a patient's control unit or clinician'sprogrammer unit, can be used to send stimulation parameters to the RFpulse generator module 106. The stimulation parameters that can becontrolled may include pulse amplitude, pulse frequency, and pulse widthin the ranges shown in Table 1. In this context the term pulse refers tothe phase of the waveform that directly produces stimulation of thetissue; the parameters of the charge-balancing phase (described below)can similarly be controlled. The patient and/or the clinician can alsooptionally control overall duration and pattern of treatment.

Stimulation Parameter Table 1 Pulse Amplitude: 0 to 20 mA PulseFrequency: 0 to 2000 Hz Pulse Width: 0 to 2 ms

The implantable neural stimulator module 114 or RF pulse generatormodule 114 may be initially programmed to meet the specific parametersettings for each individual patient during the initial implantationprocedure. Because medical conditions or the body itself can change overtime, the ability to re-adjust the parameter settings may be beneficialto ensure ongoing efficacy of the neural modulation therapy.

The programmer module 102 may be functionally a smart device andassociated application. The smart device hardware may include a CPU 206and be used as a vehicle to handle touchscreen input on a graphical userinterface (GUI) 204, for processing and storing data.

The RF pulse generator module 106 may be connected via wired connection108 to an external TX antenna 110. Alternatively, both the antenna andthe RF pulse generator are located subcutaneously (not shown).

The signals sent by RF pulse generator module 106 to the implantedstimulator 114 may include both power and parameter-setting attributesin regards to stimulus waveform, amplitude, pulse width, and frequency.The RF pulse generator module 106 can also function as a wirelessreceiving unit that receives feedback signals from the implantedstimulator module 114. To that end, the RF pulse generator module 106may contain microelectronics or other circuitry to handle the generationof the signals transmitted to the stimulator module 114 as well ashandle feedback signals, such as those from the stimulator module 114.For example, the RF pulse generator module 106 may comprise controllersubsystem 214, high-frequency oscillator 218, RF amplifier 216, a RFswitch, and a feedback subsystem 212.

The controller subsystem 214 may include a CPU 230 to handle dataprocessing, a memory subsystem 228 such as a local memory, communicationsubsystem 234 to communicate with programmer module 102 (includingreceiving stimulation parameters from programmer module), pulsegenerator circuitry 236, and digital/analog (D/A) converters 232.

The controller subsystem 214 may be used by the patient and/or theclinician to control the stimulation parameter settings (for example, bycontrolling the parameters of the signal sent from RF pulse generatormodule 106 to neural stimulator module 114). These parameter settingscan affect, for example, the power, current level, or shape of the oneor more electrical pulses. The programming of the stimulation parameterscan be performed using the programming module 102, as described above,to set the repetition rate, pulse width, amplitude, and waveform thatwill be transmitted by RF energy to the receive (RX) antenna 238,typically a dipole antenna (although other types may be used), in thewireless implanted neural stimulator module 214. The clinician may havethe option of locking and/or hiding certain settings within theprogrammer interface, thus limiting the patient's ability to view oradjust certain parameters because adjustment of certain parameters mayrequire detailed medical knowledge of neurophysiology, neuroanatomy,protocols for neural modulation, and safety limits of electricalstimulation.

The controller subsystem 214 may store received parameter settings inthe local memory subsystem 228, until the parameter settings aremodified by new input data received from the programming module 102. TheCPU 206 may use the parameters stored in the local memory to control thepulse generator circuitry 236 to generate a stimulus waveform that ismodulated by a high frequency oscillator 218 in the range from 300 MHzto 8 GHz. The resulting RF signal may then be amplified by RF amplifier226 and then sent through an RF switch 223 to the TX antenna 110 toreach through depths of tissue to the RX antenna 238.

In some implementations, the RF signal sent by TX antenna 110 may simplybe a power transmission signal used by stimulator module 114 to generateelectric pulses. In other implementations, a telemetry signal may alsobe transmitted to the stimulator module 114 to send instructions aboutthe various operations of the stimulator module 114. The telemetrysignal may be sent by the modulation of the carrier signal (through theskin if external, or through other body tissues if the pulse generatormodule 106 is implanted subcutaneously). The telemetry signal is used tomodulate the carrier signal (a high frequency signal) that is coupledonto the implanted antenna(s) 238 and does not interfere with the inputreceived on the same lead to power the implant. In one embodiment thetelemetry signal and powering signal are combined into one signal, wherethe RF telemetry signal is used to modulate the RF powering signal, andthus the implanted stimulator is powered directly by the receivedtelemetry signal; separate subsystems in the stimulator harness thepower contained in the signal and interpret the data content of thesignal.

The RF switch 223 may be a multipurpose device such as a dualdirectional coupler, which passes the relatively high amplitude,extremely short duration RF pulse to the TX antenna 110 with minimalinsertion loss while simultaneously providing two low-level outputs tofeedback subsystem 212; one output delivers a forward power signal tothe feedback subsystem 212, where the forward power signal is anattenuated version of the RF pulse sent to the TX antenna 110, and theother output delivers a reverse power signal to a different port of thefeedback subsystem 212, where reverse power is an attenuated version ofthe reflected RF energy from the TX Antenna 110.

During the on-cycle time (when an RF signal is being transmitted tostimulator 114), the RF switch 223 is set to send the forward powersignal to feedback subsystem. During the off-cycle time (when an RFsignal is not being transmitted to the stimulator module 114), the RFswitch 223 can change to a receiving mode in which the reflected RFenergy and/or RF signals from the stimulator module 114 are received tobe analyzed in the feedback subsystem 212.

The feedback subsystem 212 of the RF pulse generator module 106 mayinclude reception circuitry to receive and extract telemetry or otherfeedback signals from the stimulator 114 and/or reflected RF energy fromthe signal sent by TX antenna 110. The feedback subsystem may include anamplifier 226, a filter 224, a demodulator 222, and an A/D converter220.

The feedback subsystem 212 receives the forward power signal andconverts this high-frequency AC signal to a DC level that can be sampledand sent to the controller subsystem 214. In this way thecharacteristics of the generated RF pulse can be compared to a referencesignal within the controller subsystem 214. If a disparity (error)exists in any parameter, the controller subsystem 214 can adjust theoutput to the RF pulse generator 106. The nature of the adjustment canbe, for example, proportional to the computed error. The controllersubsystem 214 can incorporate additional inputs and limits on itsadjustment scheme such as the signal amplitude of the reverse power andany predetermined maximum or minimum values for various pulseparameters.

The reverse power signal can be used to detect fault conditions in theRF-power delivery system. In an ideal condition, when TX antenna 110 hasperfectly matched impedance to the tissue that it contacts, theelectromagnetic waves generated from the RF pulse generator 106 passunimpeded from the TX antenna 110 into the body tissue. However, inreal-world applications a large degree of variability may exist in thebody types of users, types of clothing worn, and positioning of theantenna 110 relative to the body surface. Since the impedance of theantenna 110 depends on the relative permittivity of the underlyingtissue and any intervening materials, and also depends on the overallseparation distance of the antenna from the skin, in any givenapplication there can be an impedance mismatch at the interface of theTX antenna 110 with the body surface. When such a mismatch occurs, theelectromagnetic waves sent from the RF pulse generator 106 are partiallyreflected at this interface, and this reflected energy propagatesbackward through the antenna feed.

The dual directional coupler RF switch 223 may prevent the reflected RFenergy propagating back into the amplifier 226, and may attenuate thisreflected RF signal and send the attenuated signal as the reverse powersignal to the feedback subsystem 212. The feedback subsystem 212 canconvert this high-frequency AC signal to a DC level that can be sampledand sent to the controller subsystem 214. The controller subsystem 214can then calculate the ratio of the amplitude of the reverse powersignal to the amplitude of the forward power signal. The ratio of theamplitude of reverse power signal to the amplitude level of forwardpower may indicate severity of the impedance mismatch.

In order to sense impedance mismatch conditions, the controllersubsystem 214 can measure the reflected-power ratio in real time, andaccording to preset thresholds for this measurement, the controllersubsystem 214 can modify the level of RF power generated by the RF pulsegenerator 106. For example, for a moderate degree of reflected power thecourse of action can be for the controller subsystem 214 to increase theamplitude of RF power sent to the TX antenna 110, as would be needed tocompensate for slightly non-optimum but acceptable TX antenna couplingto the body. For higher ratios of reflected power, the course of actioncan be to prevent operation of the RF pulse generator 106 and set afault code to indicate that the TX antenna 110 has little or no couplingwith the body. This type of reflected-power fault condition can also begenerated by a poor or broken connection to the TX antenna. In eithercase, it may be desirable to stop RF transmission when thereflected-power ratio is above a defined threshold, because internallyreflected power can lead to unwanted heating of internal components, andthis fault condition means the system cannot deliver sufficient power tothe implanted wireless neural stimulator and thus cannot deliver therapyto the user.

The controller 242 of the stimulator 114 may transmit informationalsignals, such as a telemetry signal, through the antenna 238 tocommunicate with the RF pulse generator module 106 during its receivecycle. For example, the telemetry signal from the stimulator 114 may becoupled to the modulated signal on the dipole antenna(s) 238, during theon and off state of the transistor circuit to enable or disable awaveform that produces the corresponding RF bursts necessary to transmitto the external (or remotely implanted) pulse generator module 106. Theantenna(s) 238 may be connected to electrodes 254 in contact with tissueto provide a return path for the transmitted signal. An A/D (not shown)converter can be used to transfer stored data to a serialized patternthat can be transmitted on the pulse modulated signal from the internalantenna(s) 238 of the neural stimulator.

A telemetry signal from the implanted wireless neural stimulator module114 may include stimulus parameters such as the power or the amplitudeof the current that is delivered to the tissue from the electrodes. Thefeedback signal can be transmitted to the RF pulse generator module 116to indicate the strength of the stimulus at the nerve bundle by means ofcoupling the signal to the implanted RX antenna 238, which radiates thetelemetry signal to the external (or remotely implanted) RF pulsegenerator module 106. The feedback signal can include either or both ananalog and digital telemetry pulse modulated carrier signal. Data suchas stimulation pulse parameters and measured characteristics ofstimulator performance can be stored in an internal memory device withinthe implanted neural stimulator 114, and sent on the telemetry signal.The frequency of the carrier signal may be in the range of at 300 MHz to8 GHz.

In the feedback subsystem 212, the telemetry signal can be downmodulated using demodulator 222 and digitized by being processed throughan analog to digital (A/D) converter 220. The digital telemetry signalmay then be routed to a CPU 230 with embedded code, with the option toreprogram, to translate the signal into a corresponding currentmeasurement in the tissue based on the amplitude of the received signal.The CPU 230 of the controller subsystem 214 can compare the reportedstimulus parameters to those held in local memory 228 to verify thestimulator(s) 114 delivered the specified stimuli to tissue. Forexample, if the stimulator reports a lower current than was specified,the power level from the RF pulse generator module 106 can be increasedso that the implanted neural stimulator 114 will have more availablepower for stimulation. The implanted neural stimulator 114 can generatetelemetry data in real time, for example, at a rate of 8 kbits persecond. All feedback data received from the implanted lead module 114can be logged against time and sampled to be stored for retrieval to aremote monitoring system accessible by the health care professional fortrending and statistical correlations.

The sequence of remotely programmable RF signals received by theinternal antenna(s) 238 may be conditioned into waveforms that arecontrolled within the implantable stimulator 114 by the controlsubsystem 242 and routed to the appropriate electrodes 254 that areplaced in proximity to the tissue to be stimulated. For instance, the RFsignal transmitted from the RF pulse generator module 106 may bereceived by RX antenna 238 and processed by circuitry, such as waveformconditioning circuitry 240, within the implanted wireless neuralstimulator module 114 to be converted into electrical pulses applied tothe electrodes 254 through electrode interface 252. In someimplementations, the implanted stimulator 114 contains between two tosixteen electrodes 254.

The waveform conditioning circuitry 240 may include a rectifier 244,which rectifies the signal received by the RX antenna 238. The rectifiedsignal may be fed to the controller 242 for receiving encodedinstructions from the RF pulse generator module 106. The rectifiersignal may also be fed to a charge balance component 246 that isconfigured to create one or more electrical pulses based such that theone or more electrical pulses result in a substantially zero net chargeat the one or more electrodes (that is, the pulses are charge balanced).The charge-balanced pulses are passed through the current limiter 248 tothe electrode interface 252, which applies the pulses to the electrodes254 as appropriate.

The current limiter 248 insures the current level of the pulses appliedto the electrodes 254 is not above a threshold current level. In someimplementations, an amplitude (for example, current level, voltagelevel, or power level) of the received RF pulse directly determines theamplitude of the stimulus. In this case, it may be particularlybeneficial to include current limiter 248 to prevent excessive currentor charge being delivered through the electrodes, although currentlimiter 248 may be used in other implementations where this is not thecase. Generally, for a given electrode having several square millimeterssurface area, it is the charge per phase that should be limited forsafety (where the charge delivered by a stimulus phase is the integralof the current). But, in some cases, the limit can instead be placed onthe current, where the maximum current multiplied by the maximumpossible pulse duration is less than or equal to the maximum safecharge. More generally, the limiter 248 acts as a charge limiter thatlimits a characteristic (for example, current or duration) of theelectrical pulses so that the charge per phase remains below a thresholdlevel (typically, a safe-charge limit).

In the event the implanted wireless neural stimulator 114 receives a“strong” pulse of RF power sufficient to generate a stimulus that wouldexceed the predetermined safe-charge limit, the current limiter 248 canautomatically limit or “clip” the stimulus phase to maintain the totalcharge of the phase within the safety limit. The current limiter 248 maybe a passive current limiting component that cuts the signal to theelectrodes 254 once the safe current limit (the threshold current level)is reached. Alternatively, or additionally, the current limiter 248 maycommunicate with the electrode interface 252 to turn off all electrodes254 to prevent tissue damaging current levels.

A clipping event may trigger a current limiter feedback control mode.The action of clipping may cause the controller to send a thresholdpower data signal to the pulse generator 106. The feedback subsystem 212detects the threshold power signal and demodulates the signal into datathat is communicated to the controller subsystem 214. The controllersubsystem 214 algorithms may act on this current-limiting condition byspecifically reducing the RF power generated by the RF pulse generator,or cutting the power completely. In this way, the pulse generator 106can reduce the RF power delivered to the body if the implanted wirelessneural stimulator 114 reports it is receiving excess RF power.

The controller 250 of the stimulator 205 may communicate with theelectrode interface 252 to control various aspects of the electrodesetup and pulses applied to the electrodes 254. The electrode interface252 may act as a multiplex and control the polarity and switching ofeach of the electrodes 254. For instance, in some implementations, thewireless stimulator 106 has multiple electrodes 254 in contact withtissue, and for a given stimulus the RF pulse generator module 106 canarbitrarily assign one or more electrodes to 1) act as a stimulatingelectrode, 2) act as a return electrode, or 3) be inactive bycommunication of assignment sent wirelessly with the parameterinstructions, which the controller 250 uses to set electrode interface252 as appropriate. It may be physiologically advantageous to assign,for example, one or two electrodes as stimulating electrodes and toassign all remaining electrodes as return electrodes.

Also, in some implementations, for a given stimulus pulse, thecontroller 250 may control the electrode interface 252 to divide thecurrent arbitrarily (or according to instructions from pulse generatormodule 106) among the designated stimulating electrodes. This controlover electrode assignment and current control can be advantageousbecause in practice the electrodes 254 may be spatially distributedalong various neural structures, and through strategic selection of thestimulating electrode location and the proportion of current specifiedfor each location, the aggregate current distribution in tissue can bemodified to selectively activate specific neural targets. This strategyof current steering can improve the therapeutic effect for the patient.

In another implementation, the time course of stimuli may be arbitrarilymanipulated. A given stimulus waveform may be initiated at a timeT_start and terminated at a time T_final, and this time course may besynchronized across all stimulating and return electrodes; further, thefrequency of repetition of this stimulus cycle may be synchronous forall the electrodes. However, controller 250, on its own or in responseto instructions from pulse generator 106, can control electrodeinterface 252 to designate one or more subsets of electrodes to deliverstimulus waveforms with non-synchronous start and stop times, and thefrequency of repetition of each stimulus cycle can be arbitrarily andindependently specified.

For example, a stimulator having eight electrodes may be configured tohave a subset of five electrodes, called set A, and a subset of threeelectrodes, called set B. Set A might be configured to use two of itselectrodes as stimulating electrodes, with the remainder being returnelectrodes. Set B might be configured to have just one stimulatingelectrode. The controller 250 could then specify that set A deliver astimulus phase with 3 mA current for a duration of 200 us followed by a400 us charge-balancing phase. This stimulus cycle could be specified torepeat at a rate of 60 cycles per second. Then, for set B, thecontroller 250 could specify a stimulus phase with 1 mA current forduration of 500 us followed by a 800 us charge-balancing phase. Therepetition rate for the set-B stimulus cycle can be set independently ofset A, say for example it could be specified at 25 cycles per second.Or, if the controller 250 was configured to match the repetition ratefor set B to that of set A, for such a case the controller 250 canspecify the relative start times of the stimulus cycles to be coincidentin time or to be arbitrarily offset from one another by some delayinterval.

In some implementations, the controller 250 can arbitrarily shape thestimulus waveform amplitude, and may do so in response to instructionsfrom pulse generator 106. The stimulus phase may be delivered by aconstant-current source or a constant-voltage source, and this type ofcontrol may generate characteristic waveforms that are static, e.g. aconstant-current source generates a characteristic rectangular pulse inwhich the current waveform has a very steep rise, a constant amplitudefor the duration of the stimulus, and then a very steep return tobaseline. Alternatively, or additionally, the controller 250 canincrease or decrease the level of current at any time during thestimulus phase and/or during the charge-balancing phase. Thus, in someimplementations, the controller 250 can deliver arbitrarily shapedstimulus waveforms such as a triangular pulse, sinusoidal pulse, orGaussian pulse for example. Similarly, the charge-balancing phase can bearbitrarily amplitude-shaped, and similarly a leading anodic pulse(prior to the stimulus phase) may also be amplitude-shaped.

As described above, the stimulator 114 may include a charge-balancingcomponent 246. Generally, for constant current stimulation pulses,pulses should be charge balanced by having the amount of cathodiccurrent should equal the amount of anodic current, which is typicallycalled biphasic stimulation. Charge density is the amount of currenttimes the duration it is applied, and is typically expressed in theunits uC/cm². In order to avoid the irreversible electrochemicalreactions such as pH change, electrode dissolution as well as tissuedestruction, no net charge should appear at the electrode-electrolyteinterface, and it is generally acceptable to have a charge density lessthan 30 uC/cm². Biphasic stimulating current pulses ensure that no netcharge appears at the electrode after each stimulation cycle and theelectrochemical processes are balanced to prevent net dc currents.Neural stimulator 114 may be designed to ensure that the resultingstimulus waveform has a net zero charge. Charge balanced stimuli arethought to have minimal damaging effects on tissue by reducing oreliminating electrochemical reaction products created at theelectrode-tissue interface.

A stimulus pulse may have a negative-voltage or current, called thecathodic phase of the waveform. Stimulating electrodes may have bothcathodic and anodic phases at different times during the stimulus cycle.An electrode that delivers a negative current with sufficient amplitudeto stimulate adjacent neural tissue is called a “stimulating electrode.”During the stimulus phase the stimulating electrode acts as a currentsink. One or more additional electrodes act as a current source andthese electrodes are called “return electrodes.” Return electrodes areplaced elsewhere in the tissue at some distance from the stimulatingelectrodes. When a typical negative stimulus phase is delivered totissue at the stimulating electrode, the return electrode has a positivestimulus phase. During the subsequent charge-balancing phase, thepolarities of each electrode are reversed.

In some implementations, the charge balance component 246 uses ablocking capacitor(s) placed electrically in series with the stimulatingelectrodes and body tissue, between the point of stimulus generationwithin the stimulator circuitry and the point of stimulus delivery totissue. In this manner, a resistor-capacitor (RC) network may be formed.In a multi-electrode stimulator, one charge-balance capacitor(s) may beused for each electrode or a centralized capacitor(s) may be used withinthe stimulator circuitry prior to the point of electrode selection. TheRC network can block direct current (DC), however it can also preventlow-frequency alternating current (AC) from passing to the tissue. Thefrequency below which the series RC network essentially blocks signalsis commonly referred to as the cutoff frequency, and in one embodimentthe design of the stimulator system may ensure the cutoff frequency isnot above the fundamental frequency of the stimulus waveform. In thisembodiment of the present invention, the wireless stimulator may have acharge-balance capacitor with a value chosen according to the measuredseries resistance of the electrodes and the tissue environment in whichthe stimulator is implanted. By selecting a specific capacitance valuethe cutoff frequency of the RC network in this embodiment is at or belowthe fundamental frequency of the stimulus pulse.

In other implementations, the cutoff frequency may be chosen to be at orabove the fundamental frequency of the stimulus, and in this scenariothe stimulus waveform created prior to the charge-balance capacitor,called the drive waveform, may be designed to be non-stationary, wherethe envelope of the drive waveform is varied during the duration of thedrive pulse. For example, in one embodiment, the initial amplitude ofthe drive waveform is set at an initial amplitude Vi, and the amplitudeis increased during the duration of the pulse until it reaches a finalvalue k*Vi. By changing the amplitude of the drive waveform over time,the shape of the stimulus waveform passed through the charge-balancecapacitor is also modified. The shape of the stimulus waveform may bemodified in this fashion to create a physiologically advantageousstimulus.

In some implementations, the wireless neural stimulator module 114 maycreate a drive-waveform envelope that follows the envelope of the RFpulse received by the receiving dipole antenna(s) 238. In this case, theRF pulse generator module 106 can directly control the envelope of thedrive waveform within the wireless neural stimulator 114, and thus noenergy storage may be required inside the stimulator itself. In thisimplementation, the stimulator circuitry may modify the envelope of thedrive waveform or may pass it directly to the charge-balance capacitorand/or electrode-selection stage.

In some implementations, the implanted neural stimulator 114 may delivera single-phase drive waveform to the charge balance capacitor or it maydeliver multiphase drive waveforms. In the case of a single-phase drivewaveform, for example, a negative-going rectangular pulse, this pulsecomprises the physiological stimulus phase, and the charge-balancecapacitor is polarized (charged) during this phase. After the drivepulse is completed, the charge balancing function is performed solely bythe passive discharge of the charge-balance capacitor, where isdissipates its charge through the tissue in an opposite polarityrelative to the preceding stimulus. In one implementation, a resistorwithin the stimulator facilitates the discharge of the charge-balancecapacitor. In some implementations, using a passive discharge phase, thecapacitor may allow virtually complete discharge prior to the onset ofthe subsequent stimulus pulse.

In the case of multiphase drive waveforms the wireless stimulator mayperform internal switching to pass negative-going or positive-goingpulses (phases) to the charge-balance capacitor. These pulses may bedelivered in any sequence and with varying amplitudes and waveformshapes to achieve a desired physiological effect. For example, thestimulus phase may be followed by an actively driven charge-balancingphase, and/or the stimulus phase may be preceded by an opposite phase.Preceding the stimulus with an opposite-polarity phase, for example, canhave the advantage of reducing the amplitude of the stimulus phaserequired to excite tissue.

In some implementations, the amplitude and timing of stimulus andcharge-balancing phases is controlled by the amplitude and timing of RFpulses from the RF pulse generator module 106, and in others thiscontrol may be administered internally by circuitry onboard the wirelessstimulator 114, such as controller 250. In the case of onboard control,the amplitude and timing may be specified or modified by data commandsdelivered from the pulse generator module 106.

FIG. 3 is a flowchart showing an example of an operation of the neuralstimulator system. In block 302, the wireless neural stimulator 114 isimplanted in proximity to nerve bundles and is coupled to the electricfield produced by the TX antenna 110. That is, the pulse generatormodule 106 and the TX antenna 110 are positioned in such a way (forexample, in proximity to the patient) that the TX antenna 110 iselectrically radiatively coupled with the implanted RX antenna 238 ofthe neural stimulator 114. In certain implementations, both the antenna110 and the RF pulse generator 106 are located subcutaneously. In otherimplementations, the antenna 110 and the RF pulse generator 106 arelocated external to the patient's body. In this case, the TX antenna 110may be coupled directly to the patient's skin.

Energy from the RF pulse generator is radiated to the implanted wirelessneural stimulator 114 from the antenna 110 through tissue, as shown inblock 304. The energy radiated may be controlled by thePatient/Clinician Parameter inputs in block 301. In some instances, theparameter settings can be adjusted in an open loop fashion by thepatient or clinician, who would adjust the parameter inputs in block 301to the system.

The wireless implanted stimulator 114 uses the received energy togenerate electrical pulses to be applied to the neural tissue throughthe electrodes 238. For instance, the stimulator 114 may containcircuitry that rectifies the received RF energy and conditions thewaveform to charge balance the energy delivered to the electrodes tostimulate the targeted nerves or tissues, as shown in block 306. Theimplanted stimulator 114 communicates with the pulse generator 106 byusing antenna 238 to send a telemetry signal, as shown in block 308. Thetelemetry signal may contain information about parameters of theelectrical pulses applied to the electrodes, such as the impedance ofthe electrodes, whether the safe current limit has been reached, or theamplitude of the current that is presented to the tissue from theelectrodes.

In block 310, the RF pulse generator 106 detects amplifies, filters andmodulates the received telemetry signal using amplifier 226, filter 224,and demodulator 222, respectively. The A/D converter 230 then digitizesthe resulting analog signal, as shown in 312. The digital telemetrysignal is routed to CPU 230, which determines whether the parameters ofthe signal sent to the stimulator 114 need to be adjusted based on thedigital telemetry signal. For instance, in block 314, the CPU 230compares the information of the digital signal to a look-up table, whichmay indicate an appropriate change in stimulation parameters. Theindicated change may be, for example, a change in the current level ofthe pulses applied to the electrodes. As a result, the CPU may changethe output power of the signal sent to stimulator 114 so as to adjustthe current applied by the electrodes 254, as shown in block 316.

Thus, for instance, the CPU 230 may adjust parameters of the signal sentto the stimulator 114 every cycle to match the desired current amplitudesetting programmed by the patient, as shown in block 318. The status ofthe stimulator system may be sampled in real time at a rate of 8 kbitsper second of telemetry data. All feedback data received from thestimulator 114 can be maintained against time and sampled per minute tobe stored for download or upload to a remote monitoring systemaccessible by the health care professional for trending and statisticalcorrelations in block 318. If operated in an open loop fashion, thestimulator system operation may be reduced to just the functionalelements shown in blocks 302, 304, 306, and 308, and the patient usestheir judgment to adjust parameter settings rather than the closedlooped feedback from the implanted device.

FIG. 4 depicts a flow chart showing an example of an operation of thesystem when the current level at the electrodes 254 is above a thresholdlimit. In certain instances, the implanted wireless neural stimulator114 may receive an input power signal with a current level above anestablished safe current limit, as shown in block 402. For instance, thecurrent limiter 248 may determine the current is above an establishedtissue-safe limit of amperes, as shown in block 404. If the currentlimiter senses that the current is above the threshold, it may stop thehigh-power signal from damaging surrounding tissue in contact with theelectrodes as shown in block 406, the operations of which are asdescribed above in association with FIG. 2.

A capacitor may store excess power, as shown in block 408. When thecurrent limiter senses the current is above the threshold, thecontroller 250 may use the excess power available to transmit a small2-bit data burst back to the RF pulse generator 106, as shown in block410. The 2-bit data burst may be transmitted through the implantedwireless neural stimulator's antenna(s) 238 during the RF pulsegenerator's receive cycle, as shown in block 412. The RF pulse generatorantenna 110 may receive the 2-bit data burst during its receive cycle,as shown in block 414, at a rate of 8 kbps, and may relay the data burstback to the RF pulse generator's feedback subsystem 212 which ismonitoring all reverse power, as shown in block 416. The CPU 230 mayanalyze signals from feedback subsystem 202, as shown in block 418 andif there is no data burst present, no changes may be made to thestimulation parameters, as shown in block 420. If the data burst ispresent in the analysis, the CPU 230 can cut all transmission power forone cycle, as shown in block 422.

If the data burst continues, the RF pulse generator 106 may push a“proximity power danger” notification to the application on theprogrammer module 102, as shown in block 424. This proximity dangernotification occurs because the RF pulse generator has ceased itstransmission of power. This notification means an unauthorized form ofenergy is powering the implant above safe levels. The application mayalert the user of the danger and that the user should leave theimmediate area to resume neural modulation therapy, as shown in block426. If after one cycle the data burst has stopped, the RF pulsegenerator 106 may slowly ramp up the transmission power in increments,for example from 5% to 75% of previous current amplitude levels, asshown in block 428. The user can then manually adjust current amplitudelevel to go higher at the user's own risk. During the ramp up, the RFpulse generator 106 may notify the application of its progress and theapplication may notify the user that there was an unsafe power level andthe system is ramping back up, as shown in block 430.

FIG. 5 is a diagram showing examples of signals that may be used todetect an impedance mismatch. As described above, a forward power signaland a reverse power signal may be used to detect an impedance mismatch.For instance, a RF pulse 502 generated by the RF pulse generator maypass through a device such as a dual directional coupler to the TXantenna 110. The TX antenna 110 then radiates the RF signal into thebody, where the energy is received by the implanted wireless neuralstimulator 114 and converted into a tissue-stimulating pulse. Thecoupler passes an attenuated version of this RF signal, forward power510, to feedback subsystem 212. The feedback subsystem 212 demodulatesthe AC signal and computes the amplitude of the forward RF power, andthis data is passed to controller subsystem 214. Similarly the dualdirectional coupler (or similar component) also receives RF energyreflected back from the TX antenna 110 and passes an attenuated versionof this RF signal, reverse power 512, to feedback subsystem 212. Thefeedback subsystem 212 demodulates the AC signal and computes theamplitude of the reflected RF power, and this data is passed tocontroller subsystem 214.

In the optimal case, when the TX antenna 110 may be perfectlyimpedance-matched to the body so that the RF energy passes unimpededacross the interface of the TX antenna 110 to the body, and no RF energyis reflected at the interface. Thus, in this optimal case, the reversepower 512 may have close to zero amplitude as shown by signal 504, andthe ratio of reverse power 512 to forward power 510 is zero. In thiscircumstance, no error condition exists, and the controller 214 sets asystem message that operation is optimal.

In practice, the impedance match of the TX antenna 204 to the body maynot be optimal, and some energy of the RF pulse 502 is reflected fromthe interface of the TX antenna 110 and the body. This can occur forexample if the TX antenna 110 is held somewhat away from the skin by apiece of clothing. This non-optimal antenna coupling causes a smallportion of the forward RF energy to be reflected at the interface, andthis is depicted as signal 506. In this case, the ratio of reverse power512 to forward power 510 is small, but a small ratio implies that mostof the RF energy is still radiated from the TX antenna 110, so thiscondition is acceptable within the control algorithm. This determinationof acceptable reflection ratio may be made within controller subsystem214 based upon a programmed threshold, and the controller subsystem 214may generate a low-priority alert to be sent to the user interface. Inaddition, the controller subsystem 214 sensing the condition of a smallreflection ratio, may moderately increase the amplitude of the RF pulse502 to compensate for the moderate loss of forward energy transfer tothe implanted wireless neural stimulator 114.

During daily operational use, the TX antenna 110 might be accidentallyremoved from the body entirely, in which case the TX antenna will havevery poor coupling to the body (if any). In this or other circumstances,a relatively high proportion of the RF pulse energy is reflected assignal 508 from the TX antenna 110 and fed backward into the RF-poweringsystem. Similarly, this phenomenon can occur if the connection to the TXantenna is physically broken, in which case virtually 100% of the RFenergy is reflected backward from the point of the break. In such cases,the ratio of reverse power 512 to forward power 510 is very high, andthe controller subsystem 214 will determine the ratio has exceeded thethreshold of acceptance. In this case, the controller subsystem 214 mayprevent any further RF pulses from being generated. The shutdown of theRF pulse generator module 106 may be reported to the user interface toinform the user that stimulation therapy cannot be delivered.

FIG. 6 is a diagram showing examples of signals that may be employedduring operation of the neural stimulator system. According to someimplementations, the amplitude of the RF pulse 602 received by theimplanted wireless neural stimulator 114 can directly control theamplitude of the stimulus 630 delivered to tissue. The duration of theRF pulse 608 corresponds to the specified pulse width of the stimulus630. During normal operation the RF pulse generator module 106 sends anRF pulse waveform 602 via TX antenna 110 into the body, and RF pulsewaveform 608 may represent the corresponding RF pulse received byimplanted wireless neural stimulator 114. In this instance the receivedpower has an amplitude suitable for generating a safe stimulus pulse630. The stimulus pulse 630 is below the safety threshold 626, and noerror condition exists. In another example, the attenuation between theTX antenna 110 and the implanted wireless neural stimulator 114 has beenunexpectedly reduced, for example due to the user repositioning the TXantenna 110. This reduced attenuation can lead to increased amplitude inthe RF pulse waveform 612 being received at the neural stimulator 114.Although the RF pulse 602 is generated with the same amplitude asbefore, the improved RF coupling between the TX antenna 110 and theimplanted wireless neural stimulator 114 can cause the received RF pulse612 to be larger in amplitude. Implanted wireless neural stimulator 114in this situation may generate a larger stimulus 632 in response to theincrease in received RF pulse 612. However, in this example, thereceived power 612 is capable of generating a stimulus 632 that exceedsthe prudent safety limit for tissue. In this situation, the currentlimiter feedback control mode can operate to clip the waveform of thestimulus pulse 632 such that the stimulus delivered is held within thepredetermined safety limit 626. The clipping event 628 may becommunicated through the feedback subsystem 212 as described above, andsubsequently controller subsystem 214 can reduce the amplitude specifiedfor the RF pulse. As a result, the subsequent RF pulse 604 is reduced inamplitude, and correspondingly the amplitude of the received RF pulse616 is reduced to a suitable level (non-clipping level). In thisfashion, the current limiter feedback control mode may operate to reducethe RF power delivered to the body if the implanted wireless neuralstimulator 114 receives excess RF power.

In another example, the RF pulse waveform 606 depicts a higher amplitudeRF pulse generated as a result of user input to the user interface. Inthis circumstance, the RF pulse 620 received by the implanted wirelessneural stimulator 14 is increased in amplitude, and similarly currentlimiter feedback mode operates to prevent stimulus 636 from exceedingsafety limit 626. Once again, this clipping event 628 may becommunicated through the feedback subsystem 212, and subsequentlycontroller subsystem 214 may reduce the amplitude of the RF pulse, thusoverriding the user input. The reduced RF pulse 604 can producecorrespondingly smaller amplitudes of the received waveforms 616, andclipping of the stimulus current may no longer be required to keep thecurrent within the safety limit. In this fashion, the current limiterfeedback may reduce the RF power delivered to the body if the implantedwireless neural stimulator 114 reports it is receiving excess RF power.

FIG. 7 is a flow chart showing a process for the user to control theimplantable wireless neural stimulator through the programmer in an openloop feedback system. In one implementation of the system, the user hasa wireless neural stimulator implanted in their body, the RF pulsegenerator 106 sends the stimulating pulse power wirelessly to thestimulator 114, and an application on the programmer module 102 (forexample, a smart device) is communicating with the RF pulse generator106. In this implementation, if a user wants to observe the currentstatus of the functioning pulse generator, as shown in block 702, theuser may open the application, as shown in block 704. The applicationcan use Bluetooth protocols built into the smart device to interrogatethe pulse generator, as shown in block 706. The RF pulse generator 106may authenticate the identity of the smart device and serialized patientassigned secure iteration of the application, as shown in block 708. Theauthentication process may utilize a unique key to the patient specificRF pulse generator serial number. The application can be customized withthe patient specific unique key through the Manufacturer Representativewho has programmed the initial patient settings for the stimulationsystem, as shown in block 720. If the RF pulse generator rejects theauthentication it may inform the application that the code is invalid,as shown in block 718 and needs the authentication provided by theauthorized individual with security clearance from the devicemanufacturer, known as the “Manufacturer's Representative,” as shown inblock 722. In an implementation, only the Manufacturer's Representativecan have access to the security code needed to change the application'sstored RF pulse generator unique ID. If the RF pulse generatorauthentication system passes, the pulse generator module 106 sends backall of the data that has been logged since the last sync, as shown inblock 710. The application may then register the most currentinformation and transmit the information to a 3rd party in a securefashion, as shown in 712. The application may maintain a database thatlogs all system diagnostic results and values, the changes in settingsby the user and the feedback system, and the global runtime history, asshown in block 714. The application may then display relevant data tothe user, as shown in block 716; including the battery capacity, currentprogram parameter, running time, pulse width, frequency, amplitude, andthe status of the feedback system.

FIG. 8 is another example flow chart of a process for the user tocontrol the wireless stimulator with limitations on the lower and upperlimits of current amplitude. The user wants to change the amplitude ofthe stimulation signal, as shown in block 802. The user may open theapplication, as show in block 704 and the application may go through theprocess described in FIG. 7 to communicate with the RF pulse generator,authenticate successfully, and display the current status to the user,as shown in block 804. The application displays the stimulationamplitude as the most prevalent changeable interface option and displaystwo arrows with which the user can adjust the current amplitude. Theuser may make a decision based on their need for more or lessstimulation in accordance with their pain levels, as shown in block 806.If the user chooses to increase the current amplitude, the user maypress the up arrow on the application screen, as shown in block 808. Theapplication can include safety maximum limiting algorithms, so if arequest to increase current amplitude is recognized by the applicationas exceeding the preset safety maximum, as shown in block 810, then theapplication will display an error message, as shown in block 812 andwill not communicate with the RF pulse generator module 106. If the userpresses the up arrow, as shown in block 808 and the current amplituderequest does not exceed the current amplitude maximum allowable value,then the application will send instructions to the RF pulse generatormodule 106 to increase amplitude, as shown in block 814. The RF pulsegenerator module 106 may then attempt to increase the current amplitudeof stimulation, as shown in block 816. If the RF pulse generator issuccessful at increasing the current amplitude, the RF pulse generatormodule 106 may perform a short vibration to physically confirm with theuser that the amplitude is increased, as shown in block 818. The RFpulse generator module 106 can also send back confirmation of increasedamplitude to the application, as shown in block 820, and then theapplication may display the updated current amplitude level, as shown inblock 822.

If the user decides to decrease the current amplitude level in block806, the user can press the down arrow on the application, as shown inblock 828. If the current amplitude level is already at zero, theapplication recognizes that the current amplitude cannot be decreasedany further, as shown in block 830 and displays an error message to theuser without communicating any data to the RF pulse generator, as shownin block 832. If the current amplitude level is not at zero, theapplication can send instructions to the RF pulse generator module 106to decrease current amplitude level accordingly, as shown in block 834.The RF pulse generator may then attempt to decrease current amplitudelevel of stimulation RF pulse generator module 106 and, if successful,the RF pulse generator module 106 may perform a short vibration tophysically confirm to the user that the current amplitude level has beendecreased, as shown in block 842. The RF pulse generator module 106 cansend back confirmation of the decreased current amplitude level to theapplication, as shown in block 838. The application then may display theupdated current amplitude level, as indicated by block 840. If thecurrent amplitude level decrease or increase fails, the RF pulsegenerator module 106 can perform a series of short vibrations to alertuser, and send an error message to the application, as shown in block824. The application receives the error and may display the data for theuser's benefit, as shown in block 826.

FIG. 9 is yet another example flow chart of a process for the user tocontrol the wireless neural stimulator 114 through preprogrammedparameter settings. The user wants to change the parameter program, asindicated by block 902. When the user is implanted with a wirelessneural stimulator or when the user visits the doctor, the Manufacturer'sRepresentative may determine and provide the patient/user RF pulsegenerator with preset programs that have different stimulationparameters that will be used to treat the user. The user will then ableto switch between the various parameter programs as needed. The user canopen the application on their smart device, as indicated by block 704,which first follows the process described in FIG. 7, communicating withthe RF pulse generator module 106, authenticating successfully, anddisplaying the current status of the RF pulse generator module 106,including the current program parameter settings, as indicated by block812. In this implementation, through the user interface of theapplication, the user can select the program that they wish to use, asshown by block 904. The application may then access a library ofpre-programmed parameters that have been approved by the Manufacturer'sRepresentative for the user to interchange between as desired and inaccordance with the management of their indication, as indicated byblock 906. A table can be displayed to the user, as shown in block 908and each row displays a program's codename and lists its basic parametersettings, as shown in block 910, which includes but is not limited to:pulse width, frequency, cycle timing, pulse shape, duration, feedbacksensitivity, as shown in block 912. The user may then select the rowcontaining the desired parameter preset program to be used, as shown inblock 912. The application can send instructions to the RF pulsegenerator module 106 to change the parameter settings, as shown in block916. The RF pulse generator module 106 may attempt to change theparameter settings 154. If the parameter settings are successfullychanged, the RF pulse generator module 106 can perform a uniquevibration pattern to physically confirm with the user that the parametersettings were changed, as shown in block 920. Also, the RF pulsegenerator module 106 can send back confirmation to the application thatthe parameter change has been successful, as shown in block 922, and theapplication may display the updated current program, as shown in block924. If the parameter program change has failed, the RF pulse generatormodule 106 may perform a series of short vibrations to alert the user,and send an error message to the application, as shown in block 926,which receives the error and may display to the user, as shown in block928.

FIG. 10 is still another example flow chart of a process for a lowbattery state for the RF pulse generator module 106. In thisimplementation, the RF pulse generator module's remaining battery powerlevel is recognized as low, as shown in block 1002. The RF pulsegenerator module 106 regularly interrogates the power supply batterysubsystem 210 about the current power and the RF pulse generatormicroprocessor asks the battery if its remaining power is belowthreshold, as shown in block 1004. If the battery's remaining power isabove the threshold, the RF pulse generator module 106 may store thecurrent battery status to be sent to the application during the nextsync, as shown in block 1006. If the battery's remaining power is belowthreshold the RF pulse generator module 106 may push a low-batterynotification to the application, as shown in block 1008. The RF pulsegenerator module 106 may always perform one sequence of short vibrationsto alert the user of an issue and send the application a notification,as shown in block 1010. If there continues to be no confirmation of theapplication receiving the notification then the RF pulse generator cancontinue to perform short vibration pulses to notify user, as shown inblock 1010. If the application successfully receives the notification,it may display the notification and may need user acknowledgement, asshown in block 1012. If, for example, one minute passes without thenotification message on the application being dismissed the applicationinforms the RF pulse generator module 106 about lack of humanacknowledgement, as shown in block 1014, and the RF pulse generatormodule 106 may begin to perform the vibration pulses to notify the user,as shown in block 1010. If the user dismisses the notification, theapplication may display a passive notification to switch the battery, asshown in block 1016. If a predetermined amount of time passes, such asfive minutes for example, without the battery being switched, theapplication can inform the RF pulse generator module 106 of the lack ofhuman acknowledgement, as shown in block 1014 and the RF pulse generatormodule 106 may perform vibrations, as shown in block 1010. If the RFpulse generator module battery is switched, the RF pulse generatormodule 106 reboots and interrogates the battery to assess powerremaining, as shown in block 1018. If the battery's power remaining isbelow threshold, the cycle may begin again with the RF pulse generatormodule 106 pushing a notification to the application, as shown in block1008. If the battery's power remaining is above threshold the RF pulsegenerator module 106 may push a successful battery-change notificationto the application, as shown in block 1020. The application may thencommunicate with the RF pulse generator module 106 and displays currentsystem status, as shown in block 1022.

FIG. 11 is yet another example flow chart of a process for aManufacturer's Representative to program the implanted wireless neuralstimulator. In this implementation, a user wants the Manufacturer'sRepresentative to set individual parameter programs from a remotelocation different than where the user is, for the user to use asneeded, as shown in block 1102. The Manufacturer's Representative cangain access to the user's set parameter programs through a secure webbased service. The Manufacturer's Representative can securely log intothe manufacturer's web service on a device connected to the Internet, asshown in block 1104. If the Manufacturer's Representative is registeringthe user for the first time in their care they enter in the patient'sbasic information, the RF pulse generator's unique ID and theprogramming application's unique ID, as shown in block 1106. Once theManufacturer's Representative's new or old user is already registered,the Manufacturer's Representative accesses the specific user's profile,as shown in block 1108. The Manufacturer's Representative is able toview the current allotted list of parameter programs for the specificuser, as shown in block 1110. This list may contain previous active andretired parameter preset programs, as shown in block 1112. TheManufacturer's Representative is able to activate/deactivate presetparameter programs by checking the box next to the appropriate row inthe table displayed, as shown in block 1114. The Manufacturer'sRepresentative may then submit and save the allotted new presetparameter programs, as shown in block 1116. The user's programmerapplication may receive the new preset parameter programs at the nextsync with the manufacturer's database.

FIG. 12 is a circuit diagram showing an example of a wireless neuralstimulator, such as stimulator 114. This example contains pairedelectrodes, comprising cathode electrode(s) 1208 and anode electrode(s)1210, as shown. When energized, the charged electrodes create a volumeconduction field of current density within the tissue. In thisimplementation, the wireless energy is received through a dipoleantenna(s) 238. At least four diodes are connected together to form afull wave bridge rectifier 1202 attached to the dipole antenna(s) 238.Each diode, up to 100 micrometers in length, uses a junction potentialto prevent the flow of negative electrical current, from cathode toanode, from passing through the device when said current does not exceedthe reverse threshold. For neural stimulation via wireless power,transmitted through tissue, the natural inefficiency of the lossymaterial may lead to a low threshold voltage. In this implementation, azero biased diode rectifier results in a low output impedance for thedevice. A resistor 1204 and a smoothing capacitor 1206 are placed acrossthe output nodes of the bridge rectifier to discharge the electrodes tothe ground of the bridge anode. The rectification bridge 1202 includestwo branches of diode pairs connecting an anode-to-anode and thencathode to cathode. The electrodes 1208 and 1210 are connected to theoutput of the charge balancing circuit 246.

FIG. 13 is a circuit diagram of another example of a wireless neuralstimulator, such as stimulator 114. The example shown in FIG. 13includes multiple electrode control and may employ full closed loopcontrol. The stimulator includes an electrode array 254 in which thepolarity of the electrodes can be assigned as cathodic or anodic, andfor which the electrodes can be alternatively not powered with anyenergy. When energized, the charged electrodes create a volumeconduction field of current density within the tissue. In thisimplementation, the wireless energy is received by the device throughthe dipole antenna(s) 238. The electrode array 254 is controlled throughan on-board controller circuit 242 that sends the appropriate bitinformation to the electrode interface 252 in order to set the polarityof each electrode in the array, as well as power to each individualelectrode. The lack of power to a specific electrode would set thatelectrode in a functional OFF position. In another implementation (notshown), the amount of current sent to each electrode is also controlledthrough the controller 242. The controller current, polarity and powerstate parameter data, shown as the controller output, is be sent back tothe antenna(s) 238 for telemetry transmission back to the pulsegenerator module 106. The controller 242 also includes the functionalityof current monitoring and sets a bit register counter so that the statusof total current drawn can be sent back to the pulse generator module106.

At least four diodes can be connected together to form a full wavebridge rectifier 302 attached to the dipole antenna(s) 238. Each diode,up to 100 micrometers in length, uses a junction potential to preventthe flow of negative electrical current, from cathode to anode, frompassing through the device when said current does not exceed the reversethreshold. For neural stimulation via wireless power, transmittedthrough tissue, the natural inefficiency of the lossy material may leadto a low threshold voltage. In this implementation, a zero biased dioderectifier results in a low output impedance for the device. A resistor1204 and a smoothing capacitor 1206 are placed across the output nodesof the bridge rectifier to discharge the electrodes to the ground of thebridge anode. The rectification bridge 1202 may include two branches ofdiode pairs connecting an anode-to-anode and then cathode to cathode.The electrode polarity outputs, both cathode 1208 and anode 1210 areconnected to the outputs formed by the bridge connection. Chargebalancing circuitry 246 and current limiting circuitry 248 are placed inseries with the outputs.

FIG. 14 is a block diagram showing an example of control functions 1405and feedback functions 1430 of a wireless implantable neural stimulator1400, such as the ones described above or further below. An exampleimplementation of the implantable neural stimulator 1400 may beimplanted lead module 114, as discussed above in association with FIG.2. Control functions 1405 include functions 1410 for polarity switchingof the electrodes and functions 1420 for power-on reset.

Polarity switching functions 1410 may employ, for example, a polarityrouting switch network to assign polarities to electrodes 254. Theassignment of polarity to an electrode may, for instance, be one of: acathode (negative polarity), an anode (positive polarity), or a neutral(off) polarity. The polarity assignment information for each of theelectrodes 254 may be contained in the input signal received by wirelessimplantable neural stimulator 1400 through Rx antenna 238 from RF pulsegenerator module 106. Because a programmer module 102 may control RFpulse generator module 106, the polarity of electrodes 254 may becontrolled remotely by a programmer through programmer module 102, asshown in FIG. 2.

Power-on reset functions 1420 may reset the polarity assignment of eachelectrode immediately on each power-on event. As will be described infurther detail below, this reset operation may cause RF pulse generatormodule 106 to transmit the polarity assignment information to thewireless implantable neural stimulator 1400. Once the polarityassignment information is received by the wireless implantable neuralstimulator 1400, the polarity assignment information may be stored in aregister file, or other short term memory component. Thereafter thepolarity assignment information may be used to configure the polarityassignment of each electrode. If the polarity assignment informationtransmitted in response to the reset encodes the same polarity state asbefore the power-on event, then the polarity state of each electrode canbe maintained before and after each power-on event.

Feedback functions 1430 include functions 1440 for monitoring deliveredpower to electrodes 254 and functions 1450 for making impedancediagnosis of electrodes 254. For example, delivered power functions 1440may provide data encoding the amount of power being delivered fromelectrodes 254 to the excitable tissue and tissue impedance diagnosticfunctions 1450 may provide data encoding the diagnostic information oftissue impedance. The tissue impedance is the electrical impedance ofthe tissue as seen between negative and positive electrodes when astimulation current is being released between negative and positiveelectrodes.

Feedback functions 1430 may additionally include tissue depth estimatefunctions 1460 to provide data indicating the overall tissue depth thatthe input radio frequency (RF) signal from the pulse generator module,such as, for example, RF pulse generator module 106, has penetratedbefore reaching the implanted antenna, such as, for example, RX antenna238, within the wireless implantable neural stimulator 1400, such as,for example, implanted lead module 114. For instance, the tissue depthestimate may be provided by comparing the power of the received inputsignal to the power of the RF pulse transmitted by the RF pulsegenerator 106. The ratio of the power of the received input signal tothe power of the RF pulse transmitted by the RF pulse generator 106 mayindicate an attenuation caused by wave propagation through the tissue.For example, the second harmonic described below may be received by theRF pulse generator 106 and used with the power of the input signal sentby the RF pulse generator to determine the tissue depth. The attenuationmay be used to infer the overall depth of wireless implantable neuralstimulator 1400 underneath the skin.

The data from blocks 1440, 1450, and 1460 may be transmitted, forexample, through Tx antenna 110 to RF pulse generator 106, asillustrated in FIGS. 1 and 2.

As discussed above in association with FIGS. 1, 2, 12, and 13, awireless implantable neural stimulator 1400 may utilize rectificationcircuitry to convert the input signal (e.g., having a carrier frequencywithin a range from about 800 MHz to about 6 GHz) to a direct current(DC) power to drive the electrodes 254. Some implementations may providethe capability to regulate the DC power remotely. Some implementationsmay further provide different amounts of power to different electrodes,as discussed in further detail below.

FIG. 15 is a schematic showing an example of a wireless implantableneural stimulator 1500 with components to implement control and feedbackfunctions as discussed above in association with FIG. 14. An RX antenna1505 receives the input signal. The RX antenna 1505 may be embedded as adipole, microstrip, folded dipole or other antenna configuration otherthan a coiled configuration, as described above. The input signal has acarrier frequency in the GHz range and contains electrical energy forpowering the wireless implantable neural stimulator 1500 and forproviding stimulation pulses to electrodes 254. Once received by theantenna 1505, the input signal is routed to power management circuitry1510. Power management circuitry 1510 is configured to rectify the inputsignal and convert it to a DC power source. For example, the powermanagement circuitry 1510 may include a diode rectification bridge suchas the diode rectification bridge 1202 illustrated in FIG. 12. The DCpower source provides power to stimulation circuitry 1511 and logicpower circuitry 1513. The rectification may utilize one or more fullwave diode bridge rectifiers within the power management circuitry 1510.In one implementation, a resistor can be placed across the output nodesof the bridge rectifier to discharge the electrodes to the ground of thebridge anode, as illustrated by the shunt register 1204 in FIG. 12.

FIG. 16 shows an example pulse waveform generated by the MFS sent to thepower management circuitry 1510 of the wireless implantable neuralstimulator 1500. This can be a typical pulse waveform generated by theRF pulse generator module 106 and then passed on the carrier frequency.The pulse amplitude is ramped over the pulse width (duration) from avalue ranging from −9 dB to +6 dB. In certain implementations, the rampstart and end power level can be set to any range from 0 to 60 dB. Thegain control is adjustable and can be an input parameter from RF pulsegenerator module 106 to the stimulation power management circuitry 1510.The pulse width, Pw, can range from 100 to 300 microseconds (μs) in someimplementations, as shown in FIG. 16. In other implementations notshown, the pulse width can be between about 5 microseconds (5 us) andabout 10 milliseconds (10 ms). The pulse frequency (rate) can range fromabout 5 Hz to 120 Hz as shown. In some implementations not shown, thepulse frequency can be below 5 Hz, and as high as about 10,000 Hz.

Returning to FIG. 15, based on the received waveform, stimulationcircuitry 1511 creates the stimulation waveform to be sent to theelectrodes 254 to stimulate excitable tissues, as discussed above. Insome implementations, stimulation circuitry 1511 may route the waveformto pulse-shaping resistor-capacitor (RC) timer 1512 to shape eachtravelling pulse waveform. An example RC-timer can be the shunt resistor1204 and smoothing resistor 1206, as illustrated in FIG. 12 and asdiscussed above. The pulse-shaping RC timer 1512 can also be used to,but is not limited to, inverting the pulse to create a pre-anodic dip orprovide a slow ramping in waveform.

Once the waveform has been shaped, the cathodic energy—energy beingtransmitted over the cathodic branch 1515 of the polarity routing switchnetwork 1523—is routed through the passive charge balancing circuitry1518 to prevent the build-up of noxious chemicals at the electrodes 254,as discussed above. Cathodic energy is then routed to input 1, block1522, of polarity routing switch network 1521. Anodic energy—energybeing transmitted over the anodic branch 1514 of the polarity routingswitch network 1523—is routed to input 2, block 1523, of polarityrouting switch network 1521. Thereafter, the polarity routing switchnetwork 1521 delivers the stimulation energy in the form of cathodicenergy, anodic energy, or no energy, to the each of the electrodes 254,depending on the respective polarity assignment, which is controlledbased on a set of bits stored in the register file 1532. The bits storedin the register file 1532 are output to a selection input 1534 of thepolarity routing switch network 1523, which causes input 1 or input 2 tobe routed to the electrodes as appropriate.

Turning momentarily to FIG. 17, a schematic of an example of a polarityrouting switch network 1700 is shown. As discussed above, the cathodic(−) energy and the anodic energy are received at input 1 (block 1522)and input 2 (block 1523), respectively. Polarity routing switch network1700 has one of its outputs coupled to an electrode of electrodes 254which can include as few as two electrodes, or as many as sixteenelectrodes. Eight electrodes are shown in this implementation as anexample.

Polarity routing switch network 1700 is configured to eitherindividually connect each output to one of input 1 or input 2, ordisconnect the output from either of the inputs. This selects thepolarity for each individual electrode of electrodes 254 as one of:neutral (off), cathode (negative), or anode (positive). Each output iscoupled to a corresponding three-state switch 1730 for setting theconnection state of the output. Each three-state switch is controlled byone or more of the bits from the selection input 1750. In someimplementations, selection input 1750 may allocate more than one bits toeach three-state switch. For example, two bits may encode thethree-state information. Thus, the state of each output of polarityrouting switch device 1700 can be controlled by information encoding thebits stored in the register 1532, which may be set by polarityassignment information received from the remote RF pulse generatormodule 106, as described further below.

Returning to FIG. 15, power and impedance sensing circuitry may be usedto determine the power delivered to the tissue and the impedance of thetissue. For example, a sensing resistor 1518 may be placed in serialconnection with the anodic branch 1514. Current sensing circuit 1519senses the current across the resistor 1518 and voltage sensing circuit1520 senses the voltage across the resistor. The measured current andvoltage may correspond to the actual current and voltage applied by theelectrodes to the tissue.

As described below, the measured current and voltage may be provided asfeedback information to RF pulse generator module 106. The powerdelivered to the tissue may be determined by integrating the product ofthe measured current and voltage over the duration of the waveform beingdelivered to electrodes 254. Similarly, the impedance of the tissue maybe determined based on the measured voltage being applied to theelectrodes and the current being applied to the tissue. Alternativecircuitry (not shown) may also be used in lieu of the sensing resistor1518, depending on implementation of the feature and whether bothimpedance and power feedback are measured individually, or combined.

The measurements from the current sensing circuitry 1519 and the voltagesensing circuitry 1520 may be routed to a voltage controlled oscillator(VCO) 1533 or equivalent circuitry capable of converting from an analogsignal source to a carrier signal for modulation. VCO 1533 can generatea digital signal with a carrier frequency. The carrier frequency mayvary based on analog measurements such as, for example, a voltage, adifferential of a voltage and a power, etc. VCO 1533 may also useamplitude modulation or phase shift keying to modulate the feedbackinformation at the carrier frequency. The VCO or the equivalent circuitmay be generally referred to as an analog controlled carrier modulator.The modulator may transmit information encoding the sensed current orvoltage back to RF pulse generator 106.

Antenna 1525 may transmit the modulated signal, for example, in the GHzfrequency range, back to the RF pulse generator module 106. In someembodiments, antennas 1505 and 1525 may be the same physical antenna. Inother embodiments, antennas 1505 and 1525 may be separate physicalantennas. In the embodiments of separate antennas, antenna 1525 mayoperate at a resonance frequency that is higher than the resonancefrequency of antenna 1505 to send stimulation feedback to RF pulsegenerator module 106. In some embodiments. antenna 1525 may also operateat the higher resonance frequency to receive data encoding the polarityassignment information from RF pulse generator module 106.

Antenna 1525 may be a telemetry antenna 1525 which may route receiveddata, such as polarity assignment information, to the stimulationfeedback circuit 1530. The encoded polarity assignment information maybe on a band in the GHz range. The received data may be demodulated bydemodulation circuitry 1531 and then stored in the register file 1532.The register file 1532 may be a volatile memory. Register file 1532 maybe an 8-channel memory bank that can store, for example, several bits ofdata for each channel to be assigned a polarity. Some embodiments mayhave no register file, while some embodiments may have a register fileup to 64 bits in size. The information encoded by these bits may be sentas the polarity selection signal to polarity routing switch network1521, as indicated by arrow 1534. The bits may encode the polarityassignment for each output of the polarity routing switch network as oneof: +(positive), −(negative), or 0 (neutral). Each output connects toone electrode and the channel setting determines whether the electrodewill be set as an anode (positive), cathode (negative), or off(neutral).

Returning to power management circuitry 1510, in some embodiments,approximately 90% of the energy received is routed to the stimulationcircuitry 1511 and less than 10% of the energy received is routed to thelogic power circuitry 1513. Logic power circuitry 1513 may power thecontrol components for polarity and telemetry. In some implementations,the power circuitry 1513, however, does not provide the actual power tothe electrodes for stimulating the tissues. In certain embodiments, theenergy leaving the logic power circuitry 1513 is sent to a capacitorcircuit 1516 to store a certain amount of readily available energy. Thevoltage of the stored charge in the capacitor circuit 1516 may bedenoted as Vdc. Subsequently, this stored energy is used to power apower-on reset circuit 1516 configured to send a reset signal on apower-on event. If the wireless implantable neural stimulator 1500 losespower for a certain period of time, for example, in the range from about1 millisecond to over 10 milliseconds, the contents in the register file1532 and polarity setting on polarity routing switch network 1521 may bezeroed. The wireless implantable neural stimulator 1500 may lose power,for example, when it becomes less aligned with RF pulse generator module106. Using this stored energy, power-on reset circuit 1540 may provide areset signal as indicated by arrow 1517. This reset signal may causestimulation feedback circuit 1530 to notify RF pulse generator module106 of the loss of power. For example, stimulation feedback circuit 1530may transmit a telemetry feedback signal to RF pulse generator module106 as a status notification of the power outage. This telemetryfeedback signal may be transmitted in response to the reset signal andimmediately after power is back on neural stimulator 1500. RF pulsegenerator module 106 may then transmit one or more telemetry packets toimplantable wireless neutral stimulator. The telemetry packets containpolarity assignment information, which may be saved to register file1532 and may be sent to polarity routing switch network 1521. Thus,polarity assignment information in register file 1532 may be recoveredfrom telemetry packets transmitted by RF pulse generator module 106 andthe polarity assignment for each output of polarity routing switchnetwork 1521 may be updated accordingly based on the polarity assignmentinformation.

The telemetry antenna 1525 may transmit the telemetry feedback signalback to RF pulse generator module 106 at a frequency higher than thecharacteristic frequency of an RX antenna 1505. In one implementation,the telemetry antenna 1525 can have a heightened resonance frequencythat is the second harmonic of the characteristic frequency of RXantenna 1505. For example, the second harmonic may be utilized totransmit power feedback information regarding an estimate of the amountof power being received by the electrodes. The feedback information maythen be used by the RF pulse generator in determining any adjustment ofthe power level to be transmitted by the RF pulse generator 106. In asimilar manner, the second harmonic energy can be used to detect thetissue depth. The second harmonic transmission can be detected by anexternal antenna, for example, on RF pulse generator module 106 that istuned to the second harmonic. As a general matter, power managementcircuitry 1510 may contain rectifying circuits that are non-lineardevice capable of generating harmonic energies from input signal.Harvesting such harmonic energy for transmitting telemetry feedbacksignal could improve the efficiency of wireless implantable neuralstimulator 1500. FIGS. 18A and 18B and the following discussiondemonstrate the feasibility of utilizing the second harmonic to transmittelemetry signal to RF pulse generator module 106.

FIGS. 18A and 18B respectively show an example full-wave rectified sinewave and the corresponding spectrum. In particular, a full-waverectified 915 MHz sine wave is being analyzed. In this example, thesecond harmonic of the 915 MHz sine wave is an 1830 MHz output harmonic.This harmonic wave may be attenuated by the amount of tissue that theharmonic wave needs to pass through before reaching the externalharmonic receiver antenna. In general, an estimation of the power levelsduring the propagation of the harmonic wave can reveal the feasibilityof the approach. The estimation may consider the power of received inputsignal at the receiving antenna (e.g., at antenna 1505 and at 915 MHz),the power of the second harmonic radiated from the rectified 915 MHzwaveform, the amount of attenuation for the second harmonic wave topropagate through the tissue medium, and an estimation of the couplingefficiency for the harmonic antenna. The average power transmitted inWatts can be estimated by Equation 1:

Pt=Pk DuC

P _(r)=(P _(t) /A _(ant))(1−{Γ}²)Lλ ² G _(r)η/4π)  (1)

Table 1 below tabulates the denotations of each symbol and thecorresponding value used in the estimation.

TABLE 1 Parameters utilized in development of the Received Powerequation. Parameter Value P_(k) (Peak Power)(W) 1.576 DuC (Duty Cycle)0.5 P_(t) (Average power transmitted) 1.576 A_(ant) (Antenna aperturearea) (m²) 0.01 Γ (Voltage reflection coefficient) 0.5 1 − {Γ}²(Transmission Loss) 0.75 L (Loss through tissue) (dB) 10 λ (Wavelength)(m) 0.689 G_(r) (Gain of implanted receiving antenna) 2 η (RF-DCefficiency) 0.5 R_(torso)(Equivalent Tissue Resistance) (Ohm) 500

In estimating L, the loss due to the attenuation in the tissue,attenuations from the fundamental (for the forward path to the implantedlead module 114) and second harmonics (for the reverse path from theimplanted lead module 113) may be considered. The plane wave attenuationis given by the following equation (2) and Table 2:

$\alpha = {\frac{2\pi \; f}{c}\left( \frac{ɛ_{r}}{2} \right)^{0.5}\left( {{- 1} + \left( {1 + \left( \frac{\sigma}{{\varpi ɛ}_{0}ɛ_{r}} \right)^{2}} \right)^{0.5}} \right)^{0.5}}$

where

f=frequency

c=speed of light in vacuum

ε_(r)=relative dielectric constant

σ=conductivity

ε₀=permittivity of vacuum

TABLE 2 Output power loss for 915 MHz and 1830 MHz harmonic at 1 cmdepth. Freq(MHz) □r □□□S/m) □□neper/m) Power loss 0.915e9 41.329 0.8716925.030 0.606  1.83e9 38.823 1.1965 35.773 0.489

The worst case assumption for coupling of the harmonics wave to theexternal receive antenna is that the power radiated at the harmonicfrequency by the implanted telemetry antenna (e.g., telemetry antenna1625) is completely absorbed by external receive antenna. This worstcase scenario can be modeled by the following equation (3) and Table 3:

P _(nr) =P _(t) L _(n) L _(na)  (3)

where

n=nth Harmonic

P_(nr)=nth Harmonic Antenna Received Power (W)

P_(t)=Total Received power of Implant(W)

L_(n)=Power of nth Harmonic ofImplantPower(W)

L_(na)=Attenuation Loss Factor

TABLE 3 Output total power and received harmonic power for the 2^(nd)harmonic. P_(t)(W) L_(n) L_(na) P_(nr)(W) dBm 0.356 .2421 0.489 0.042216.3

In sum, the reduction of power levels has been estimated to be about 10dB utilizing these developed equations. This includes the attenuation ofa 915 MHz plane wave that propagates through tissue depths from 1 cm to6 cm. The average received power, Pr, at 915 MHz is 0.356 W. The powerin the second harmonic (1830 MHz) is about −6.16 dB, as obtained from aSPICE simulation using a full wave rectified 915 MHz sine wave. Theestimate of 10 dB means a reduction of a factor of 10, which isacceptable for field operations. Thus, the feasibility of utilizing thesecond harmonic frequency to transmit the telemetry feedback signal backto the RF pulse generator module 106 has been demonstrated.

FIG. 19 is a flow chart illustrating an example of operations of controland feedback functions of the neural stimulator. The operations aredescribed with respect to the wireless implantable neural stimulator1500, although the operations may be performed by other variations of awireless implantable neural stimulator, such as the ones describedabove.

RF pulse generator module 106 transmits one or more signals containingelectrical energy (1900). RF pulse generator module 106 may also beknown as a microwave field stimulator (MFS) in some implementations. Thesignal may be modulated at a microwave frequency band, for example, fromabout 800 MHz to about 6 GHz.

The input signal containing electrical energy is received by RX antenna1505 of the neural stimulator 1500 (1910). As discussed above, RXantenna 1505 may be embedded as a dipole, microstrip, folded dipole orother antenna configuration other than a coiled configuration.

The input signal is rectified and demodulated by the power managementcircuitry 1510, as shown by block 1911. Some implementations may providewaveform shaping and, in this case, the rectified and demodulated signalis passed to pulse shaping RC timer (1912). Charge balancing may beperformed by charge balancing circuit 1518 to provide a charged balancedwaveform (1913). Thereafter, the shaped and charge balanced pulses arerouted to electrodes 254 (1920), which deliver the stimulation to theexcitable tissue (1921).

In the meantime, the current and voltage being delivered to the tissueis measured using the current sensor 1519 and voltage sensor 1520(1914). These measurements are modulated and amplified (1915) andtransmitted to the RF pulse generator module 106 from telemetry antenna1525 (1916). In some embodiments, the telemetry antenna 1525 and RXantenna 1505 may utilize the same physical antenna embedded within theneural stimulator 1500. The RF pulse generator module 106 may use themeasured current and voltage to determine the power delivered to thetissue, as well as the impedance of the tissue.

For example, the RF pulse generator module 106 may store the receivedfeedback information such as the information encoding the current andvoltage. The feedback information may be stored, for instance, as apresent value in a hardware memory on RF pulse generator module 106.Based on the feedback information, RF pulse generator module 106 maycalculate the impedance value of the tissue based on the current andvoltage delivered to the tissue.

In addition, RF pulse generator module 106 may calculate the powerdelivered to the tissue based on the stored current and voltage (1950).The RF pulse generator module 106 can then determine whether power levelshould be adjusted by comparing the calculated power to the desiredpower stored, for example, in a lookup table stored on the RF pulsegenerator module 106 (1917). For example, the look-up table may tabulatethe optimal amount of power that should be delivered to the tissue forthe position of the receive antenna 1505 on neural stimulator 1500relative to the position of the transmit antenna on RF pulse generatormodule 106. This relative position may be determined based on thefeedback information. The power measurements in the feedback informationmay then be correlated to the optimal value to determine if a powerlevel adjustment should be made to increase or decrease the amplitude ofstimulation of the delivered power to the electrodes. The power leveladjustment information may then enable the RF pulse generator module 106to adjust parameters of transmission so that the adjusted power isprovided to the RX antenna 1505.

In addition to the input signal containing electrical energy forstimulation, the RF pulse generator module 106 may send an input signalthat contains telemetry data such as polarity assignment information(1930). For instance, upon power on, the RF pulse generator module 106may transmit data encoding the last electrode polarity settings for eachelectrode before RF pulse generator module 106 was powered off. Thisdata may be sent to telemetry antenna 1525 as a digital data streamembedded on the carrier waveform. In some implementations, the datastream may include telemetry packets. The telemetry packets are receivedfrom the RF pulse generator module 106 and subsequently demodulated(1931) by demodulation circuit 1531. The polarity setting information inthe telemetry packets is stored in the register file 1532 (1932). Thepolarity of each electrode of electrodes 254 is programmed according tothe polarity setting information stored in the register file 1532(1933). For example, the polarity of each electrode may be set as oneof: anode (positive), cathode (negative), or neutral (off).

As discussed above, upon a power-on reset, the polarity settinginformation is resent from the RF pulse generator module 106 to bestored in the register file 1532 (1932). This is indicated by the arrow1932 to 1916. The information of polarity setting stored in the registerfile 1532 may then be used to program the polarity of each electrode ofelectrodes 254 (1933). The feature allows for re-programming of apassive device remotely from the RF pulse generator module 106 at thestart of each powered session, thus obviating the need of maintainingCMOS memory within the neural stimulator 1500.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. A system for modulating neural tissue in apatient comprising: an implantable neural stimulator comprising one ormore electrodes, at least one antenna and an electrode interface; aradio-frequency (RF) pulse generator module comprising an antenna moduleconfigured to send an input signal to the antenna in the implantableneural stimulator through electrical radiative coupling, the inputsignal containing electrical energy and polarity assignment informationthat designates polarity assignments of the electrodes in theimplantable neural stimulator; and wherein the implantable neuralstimulator is configured to: control the electrode interface such thatthe electrodes have the polarity assignments designated by the polarityassignment information, create one or more electrical pulses suitablefor modulation of neural tissue using the electrical energy contained inthe input signal, and supply the electrical pulses to the electrodesthrough the electrode interface such that the electrodes apply theelectrical pulses to the neural tissue with the polarity assignmentsdesignated by the polarity assignment information.